This invention relates to a method for producing a low molecular weight polymer by polymerizing or copolymerizing a conjugated diolefin in the presence of an inert hydrocarbon solvent a specific chain transfer agent and a polymerization initiator selected from metallic sodium and organic sodium compounds in which method the molecular weight of obtained polymer or copolymer can be controlled reproducibly. More particularly, the method is characterized in that oxygen is fed into the polymerization vessel when polymerization or copolymerization is carried out by successively introducing the solvent, chain transfer agent, polymerization initiator and monomer or monomers into the polymerization vessel.
Liquid low polymers which are made by polymerizing conjugated dienes such as butadiene and isoprene have been hitherto used for a wide variety of purposes, for example, thermosetting resins, synthetic drying oils, sealing agents, baking paints, rubber additives and various chemical intermediates, since such liquid low polymers have many unsaturated bonds in their molecules. However, it is necessary to employ a polymer having the most suitable fluidity when it is used for each purpose. In other words, a liquid low molecular weight polymer which has a suitable viscosity must be used for each purpose. The fluidity of a liquid low polymer depends predominately upon the molecular weight of the polymer so that it has long been desired to keep the molecular weight of the liquid low polymer under exact control.
As the method for controlling molecular weight, for example, what is called living polymerization is known in which polycyclic aromatic compound sodium complex is used as a polymerization initiator in a polar solvent such as tetrahydrofuran.
In this method, however, only one molecular of a polymer is produced per one molecule of a polymerization initiator. A quite large quantity of a initiator is required for the preparation of a low molecular weight polymer, and further, a relatively expensive solvent must be employed in the reaction.
It is well known that toluene or the like is used as a chain transfer agent when a conjugated diene such as butadiene is polymerized in the presence of a polymerization initiator such as metallic sodium or organic sodium compounds. In this method, the low molecular weight polymer is produced by chain transfer reaction with the toluene. Further, this method is advantageous in that the quantity of a polymerization initiator used is small and the raw materials for the reaction are inexpensive as compared with the above-mentioned living polymerization method.
In accordance with the latter method, a polymer having a number average molecular weight of 300 to 10,000 can be produced easily by controlling the concentrations of polymerization initiator, chain transfer agent and monomer.
The present inventors have found out, however, that the molecular weight of the obtained polymer cannot be well controlled with reproducibility when the above polymerization is carried out successively in a relatively large-scaled equipment.
That is, when the polymerization is carried out at given raw material concentration, at a given temperature, pressure and stirring condition in a sealed polymerization vessel it was found that the average molecular weight or the viscosity of reaction product sometimes rose abruptly. These phenomena will be shown in the Comparative Examples described below. Therefore, the reproducible production of polymers having well controlled molecular weights cannot be expected.